import from collections will not work with Python 3.8

Get warnings now
```
/usr/local/lib/python3.7/site-packages/poll/__init__.py:268: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
if isinstance(ex, collections.Iterable):

-- Docs: https://docs.pytest.org/en/latest/warnings.html
```

Start at poll/__init__.py around line 268, where the warning identifies use of collections.Iterable. Check the project’s supported Python versions and run its existing tests under Python 3.8; done means the deprecated import warning is gone and the tests still pass.

Written by the indexing model from the issue text.